1. Field of the Invention
The invention relates to a device for cooling bent glass sheets. More specifically, the invention relates to a device for cooling glass sheets on the run, that is to say in which the glass sheets are advancing while they are being cooled. Although the invention is not limited to such devices, it will be more particularly described with reference to techniques for bending and for toughening glass sheets which are running along a roller conveyor having a profile that is curved in the direction in which the glass sheets run.
2. Discussion of the Background
The abovementioned techniques are known, especially from French Patents FR-B-2,242,219 and FR-B-2,549,465, and consist in making glass sheets, heated in a horizontal furnace, run between two layers of rollersxe2x80x94or other rotating elementsxe2x80x94arranged in a curvilinear profile and passing through a terminal toughening zone. For the production of side windows, sun-roofs or other windows, especially of cylindrical shape, the layers consist, for example, of straight cylindrical rods arranged in a circular profile. The layers may also consist of elements giving the windows a second curvature, such as conical elements or else of the diabolo/barrel type. This technique makes it possible to achieve a very high production capacity since, on the one hand, the glass sheets do not have to be widely spaced, it being possible for one glass sheet to enter the forming zone without any problem while the treatment for preceding sheet has not been completed and, on the other hand, if the length of the rollers allows it, two or three glass sheets arranged transversely may be treated simultaneously.
The run speed of the glass sheets, or plates is at least equal to 10 cm/s and is about 15 to 25 cm/s. The speed normally does not exceed 30 cm/s in order to allow a sufficient toughening time.
Under relatively standard blowing conditions and for a 3.2 mm thick glass sheet, having to meet the requirements of European Regulation No. 43 relating to the homologation of safety-glass windows and materials for windows intended to be fitted into motor vehicles and their trailers, the techniques described above are completely satisfactory. According to the requirements of the abovementioned regulation, the toughening stresses must be such that the window, should it break, does so into a number of fragments which, in any 5xc3x975 cm square, is neither less than 40 nor greater than 350 (this number increases to 400 in the case of windows having a thickness of less than or equal to 2.5 mm). Still according to these requirements, no fragment must be greater than 3.5 cm, except possibly in a strip 2 cm in width at the periphery of the window and within a 7.5 cm radius around the point of impact, and there must be no elongate fragment with a length greater than 7.5 cm.
When the thickness of the glass sheets decreases, and in order to meet the same toughening standards, the heat exchange coefficient must be greatly increased. With regard to standard toughening plants, i.e. plants having nozzles of a given diameter, the heat exchange coefficient is improved by increasing the air flow rate, which leads to a greater velocity of the air around the glass sheets.
Such a construction firstly has the drawback of requiring boosters or new, more powerful, plants in order to produce the required air flow rates, these being very expensive. Moreover, this results in local overpressures and confinement of the air which cannot very easily escape, especially on the upper face in the case of a sheet running on an upwardly concave conveyor. Such a confinement then results, to the contrary, in a decrease in the heat exchange coefficient.
Another solution consists in decreasing the diameter of the nozzles in order to increase the velocity of the air at a constant flow rate. In such a case, the decrease in diameter of the nozzles means that the orifices have to be closer to the glass sheets in order to maintain the required velocities at the surface of the said glass sheets. However, to obtain such a result, it is necessary to use very long nozzles which result in very large pressure drops unacceptable from an industrial standpoint, especially because of
The object of the invention is thus to provide a novel device for cooling bent glass sheets which is more flexible from a use standpoint than the current techniques and which makes it possible to increase the heat exchange coefficient while obviating the abovementioned drawbacks and, more particularly, without requiring fundamentally different and expensive plants.
This object is achieved according to the invention by a device for cooling bent glass sheets on a roller conveyor, the said device comprising blowing boxes inserted between the rollers and having a surface opposite the glass sheet at a distance from the latter of less than 30 mm and preferably of less than 10 mm, the said surface being drilled with several holes from which the air is driven towards the glass sheet.
Rollers should be understood to mean any type of axisymmetric element which, because of its shape and/or its arrangement, can give the glass sheets a curvature. They are, for example, cylinders, devices of the diabolo/barrel type, conical systems and hogged systems, especially such as those described in Patents EP-B-0,263,030 and EP-B-0,474,531.
Such a device according to the invention allows an effective increase in the heat exchange coefficient while retaining the basic cooling plants and not requiring boosters, especially for the purpose of obtaining higher blown air flow rates. Since the blown air flow rates have not been increased with respect to the usual operating conditions, the risks of local overpressure and therefore of air confinement are avoided. Moreover, the construction of boxes having a plate drilled with holes considerably limits the pressure drops, especially with respect to the flow of air in the tube forming a blowing nozzle.
According to a preferred embodiment of the invention, especially in the case of toughening bent glass sheets, the boxes are positioned above and below the path of the glass sheets.
Preferably again, the diameters of the holes are between 2 and 8 mm and advantageously less than 5 mm; they are distributed with a pitch of less than 20 mm and preferably of between 3 and 6 mm. This preferred embodiment not only allows the heat exchange coefficient to be increased with respect to the usual cooling techniques, especially for a given air flow rate, but it also allows more homogeneous distribution of the cooling at the surface of a glass sheet. This is because, compared with the usual cooling plants, the blowing orifices are closer together and result in greater blowing homogeneity at the surface of a glass sheet. The usual cooling plants consist of nozzles distributed with a pitch of generally greater than 30 mm, which results in acceptable cooling of the glass surface from a results standpoint, but the cooling is markedly less homogeneous than in the device proposed by the invention.
According to an advantageous embodiment of the invention, the device, combined with a blowing pressure of less than 3000 mm of water column, allows a coefficient of heat exchange with the glass of at least 800 W/m2xc2x7K and preferably of at least 1000 W/m2xc2x7K. The current techniques, although they are not associated with very expensive means, such as boosters, cannot exceed a heat exchange coefficient of about 800 W/m2xc2x7K.
According to an alternative form of the invention, the device is used at the start of the cooling zone, the rest of the zone remaining in a standard configuration. In this way, the glass sheets run along a conveyor and are cooled in two phases, the heat exchange coefficient being greater during the first phase.
Such an embodiment is particularly advantageous in the case of toughening the glass; this is because it allows a high heat exchange coefficient at the start of toughening and a lower one thereafter. The inventors have evidence for the fact that, for toughening a given glass sheet, modulated toughening consisting of a high heat exchange coefficient at the start, and then a lower one afterwards, leads to results from a toughening standpoint that are superior to those obtained with a constant heat exchange coefficient, for the same overall energy costs.
According to other alternative forms of the invention, it is possible to provide differential cooling of a glass sheet between its two faces; in particular, it is in fact possible to equip the cooling unit with a device according to the invention opposite only one face of the glass sheets, the other face being cooled by standard devices. In this way, it is possible to obtain a higher heat exchange coefficient for one of the two faces. Especially in the case of toughening, such a technique allows the curvature of the bent glass sheets to be modified.
In order to achieve similar results, the invention also proposes to differentiate the heat exchange coefficient on the same surface at a given instant. To do this, the invention advantageously provides for that surface of the boxes which is opposite one face of the glass to be drilled with holes which are not all the same size. According to a preferred embodiment of the invention, the blowing holes are holes of varying aperture; these may be obtained by movable elements which partially or completely mask a hole.
According to a preferred embodiment of the invention, the rollers of the conveyor are positioned in an upwardly concave curved profile. The device thus described according to the invention is advantageously intended for toughening bent glass sheets on plants of the type described in French Patents FR-B-2,242,219 and FR-B-2,549,465. In the technique described in these documents, the rollers usually have diameters of between 20 and 30 mm and are spaced apart with a pitch of between 40 and 130 mm. It was not obvious to insert into these plants devices such as those of the invention which, especially in the case of small diameter holes, must be very close to the surfaces of the glass sheets, for fear of the problem of extracting the blown air. Tests have shown that, despite the closeness of the blowing holes to the glass sheets, the air can be properly extracted, resulting in heat exchange coefficients that are higher than those obtained with the standard toughening plants.
The use of a device according to the invention in this type of plant makes it possible to bend and toughen glass sheets that are thinner than those normally toughened in this type of plant. This is because the minimum thickness of glass sheet that the current plants usually allow to be toughened is 3.15 mm, which corresponds to a heat exchange coefficient of approximately 600 W/m2xc2x7K. Without any modification, the device according to the invention allows glass sheets with a thickness of 2.85 mm or even 2.5 mm to be toughened, this corresponding to a heat exchange coefficient which may be as much as 1000 W/m2xc2x7K.
The invention also makes it possible to improve, especially from a productivity standpoint, the results usually obtained for toughening glass sheets having holes which pass through their thickness. It is quite often the case for there to be holes of this type in glass sheets intended for equipping motor vehicles and for being fitted in a sliding manner, in order to allow opening.
The device according to the invention also makes it possible to improve the optical quality of the glass sheets, this device allowing more homogeneous blowing distribution than the usual techniques. This improvement in optical quality of the glass sheets is even greater for the production of laminated glazing assemblies. This is because combining two glass sheets, or more generally two sheets of transparent materials, does not mean that their defects from an optical standpoint are added, rather it increases them much more.
The DIN 32 305 standard defines an xe2x80x9coptical qualityxe2x80x9d for side windows intended for the motor-vehicle industry.
The invention thus makes it possible to produce laminated glazing assemblies comprising at least one glass sheet having surface stresses of greater than 60 MPa, the said laminated glazing assembly complying with the DIN 32 305 standard.
In particular, the invention allows the production of a laminated glazing assembly comprising at least one toughened glass sheet having surface stresses of greater than 100 MPa and preferably of about 130 MPa, the said laminated glazing assembly complying with the DIN 32 305 standard.
A laminated glazing assembly of this type is especially advantageous for impact-resistance functions. These laminated glazing assemblies may comprise one or more glass sheets, whatever their thicknesses. However, the invention also makes it possible to produce such laminated glazing assemblies using glass sheets with a thickness of less than 3 mm and preferably of less than 2 mm, it being possible for the said laminated glazing assemblies to have the thickness of the monolithic glazing assemblies currently used in motor vehicles.